So has the Audi A4 at the local audi garage ? The markets slow as any car dealer will tell you. Its desperate sales that push prices down as people need to get rid. Im selling, but in no means desperate, and weighing it up, there is no point getting rid of a car for stupid money that is a joy to drive , when in the summer the price inevitably goes up. Its always the same, sports cars in the recession fall the worst but its the constant panic that forces the prices down.
Z4 roadsters are the same everyone who has too gets rid in the winter, everyone who wants too gets rid in the spring.
In terms of the duration of sale, you will notice that most Z4's have been for sale for that long. I have had 2 sales now, both have fallen through due to job losses.